The present invention relates generally to demand assignment time division multiple access (TDMA) communication systems, and more specifically to the detection of a fault in a central station receive channel of a demand assignment TDMA communication system.
In a demand assignment TDMA communication system, a central station periodically transmits a polling signal to elicit a response from each of a plurality of geometrically scattered terminal stations. If there is a packet to transmit, each of the polled terminal stations transmits it on an assigned channel, or time slot, to the central station. If required, a repeater station is provided to relay signals between remote terminal stations and the central station. As shown and described in Japanese Provisional Patent Specification (Tokkaisho) 61-281729, a prior art demand assignment TDMA communication system employs a fault detection scheme in which the central station utilizes the response from the polled station as an indication of an operating condition of its receive channels to effect switching from a regular receive channel to a spare receive channel if a faulty condition is detected in the regular channel. The repeater station employs the same fault detection scheme to provide channel switching. However, the response from a remote terminal station occurs once for every N frames, where N usually corresponds to the number of maximum terminal stations that can be accommodated in the TDMA system. Thus, if a trouble should develop in the repeater station, the prior art fault detection system is not capable of detecting it until the remote station is polled next time which is N frames later than the time of occurrence of the trouble, and hence the downtime of the repeater station can be as long as N frame intervals. The amount of this loss time could be substantial to the TDMA system because of its capability for carrying high density information. A similar situation occurs in the central station of a demand assignment TDMA system in which the number of terminal stations registered, or actually operating in the system is much smaller than the system capacity since no response is returned during frames which are assigned to nonregistered terminal stations.